Polluted sites exist in all industrialized countries. When referring to contaminated sites, this means contaminated locations or lands, on which, in the past, pollutants were introduced into the media, such as soils, in quantities that are dangerous for the soil, the phreatic table and/or their users.
The problem of polluted soils is real and the treatment of these soils is an expanding field. The challenges in markets that are based on decontamination are a function of the contaminants that are present, the types of terrains, the possibility of economic reclamation of a terrain, sanitary aspects vis-à-vis the neighboring populations and the harm caused to the environment.
Contaminants are numerous and diverse. While some have been known for a long time, such as metals and petroleum hydrocarbons, others have only recently recognized, as they resulted from activities at the end of the 20th century. This is the case, for example, of pesticides, PCBs, dioxins, furanes, PAHs, etc.
Studies into the treatment of contaminated substances are numerous for various media (soils, sediments, sewage, industrial waste, etc.) and by various modes of ex situ or in situ treatment (biological, physio-chemical, thermal). While many processes have been commercialized, very few propose the treatment of organic and inorganic contaminants. There thus remains a significant portion of soils that display mixed contamination. Presently, the treatment of these contaminants relies on the sequential application of decontamination processes, thus considerably increasing the costs and limiting the treatment of media displaying mixed contamination. Other challenges related to the decontamination of mixed media result from fundamental differences in the properties of the targeted contaminants (organic and inorganic) as well as in the heterogeneity of the distribution of the organic contaminants in the matrix.
For media presenting mixed and heterogeneous contamination, such as metals and hydrophobic organic compounds like polycyclic aromatic hydrocarbons (PAH), certain treatment methods have been explored.
Soil washing is an ex situ process based on the idea of rinsing with water to remove the contaminants (“ungluing” them from the soil matrix and transferring them to a concentrated side phase). The extraction process of the organic and/or inorganic contaminants of the soil can be done in one of the two following ways: by dissolving or transferring them to the washing solution; by concentrating them to a smaller volume of soil by particle separation, gravimetric, flotation separation and/or attrition.
Soil washing is a chemical process often associated with metallurgical processes. This treatment technique is a pollutant transfer technology, which requires an upstream treatment of the contaminants. It applies principally to soils, sediments, rocks and sewage sludge. Its principle is relatively simple. The contaminants adsorbed on the fine soil particles are separated and extracted from the bulk of soil. The washing capacity of water is increased by the addition of various agents and additives.
As concerns soils polluted by organic compounds, the washing is often performed with the help of surfactants. The majority of the studies pertaining to surfactants and washing contaminated soils utilize pure surfactants. Certain studies have demonstrated the superior action of surfactant mixtures for rehabilitating soils. Other studies have been conducted to compare the extraction efficacy of several surfactants vis-à-vis PAH compounds contained in contaminated soils in the presence of a salt or not. In the study conducted by Lopez entitled, “Washing of soil contaminated with PAHs and heavy petroleum fractions using two anionic and one ionic surfactant: Effect salt addition” (Journal of Environmental Science and Health Part a-Toxic/Hazardous Substances & Environmental Engineering 2005; 40:1107), Tween 80 appears as the agent enabling the best PAH solubilization and addition of NaCl enables increased efficacy of the SDS (until then considered as the least efficient of the surface agents used).
Washing of PAHs using surfactants mostly employs anionic or non-ionic surface actives or mixes thereof, rather than other types of surfactants.
As concerns soils polluted with metals, the washing of metals can be done using acids, chelating agents, biosurfactants or oxydants. The agents that are most widely used remain, however, acids and chelating agents. During the process, the metals are solubilized, then a solid-liquid separation (e.g. filtration) enables obtaining the metals in the liquid and decontaminated soils.
In “Pb mobility and extractant optimization for contaminated soil” (Environmental Progress, 1997; 16:88), Legiec compared the efficiency of acetic acid, citric acid, hydrochloric acid and caustic soda, as well as sodium chloride as a leaching agent. He showed that the use of HCl, coupled with NaCl 0.5 or 1.0 M gave the best efficiency for removing lead.
For soils presenting mixed and heterogeneous contamination, it was necessary to proceed to a treatment including several washing cells, each being adapted to a type of contaminant. The key for an efficient soil washing using such a method results from the arrangement and configuration of the processing units, as well as the characterization and the comprehension of the pollutant/soil interactions. The candidate contaminants for soil washing are principally metals, semi-volatile organics, PAHs, pesticides, PCBs, and slightly radioactive elements. While washing PAHs is generally associated with the action of a surface active, the washing of metals can be realized by using various agents: pH, chelating agents, biosurfactants, oxidants.
Khodadoust et al. describe in “Effect of different extraction agents on metal and organic contaminant removal from a field soil” (Journal of Hazardous Materials 2005; 117:15) the extraction of PAH phenanthrene, lead, and zinc from a contaminated soil using diverse additives including surfactants, co-solvents and acids. Khodadoust et al. find that two sequential extractions are necessary to remove the inorganic and organic contaminants from the soil. Of all these sequential extractions, phenanthrene is extracted only in the presence of surfactants. A first washing using EDTA followed by a subsequent washing using Tween 80 surfactant, or an initial washing using Tween 80 followed by a subsequent washing using citric acid 1 M, are the two sequential conditions enabling a satisfactory extraction. Khodadoust et al. conclude that the sequential washings using a different additive for each wash seems to be necessary for the treatment of soils contaminated with metals and organic compounds.
A study by Semer et al. entitled “Evaluation of soil washing process to remove mixed contaminants from sandy loam” (Journal of Hazardous Materials, 1996; 45:45) looks at a process for washing soil using a solution of sulphuric acid 1N and isopropanol.
There are also other processes for decontaminating polluted soils using the techniques of flotation, screening, and/or hydrocycloning.
The known methods for treating media polluted by both metals and hydrophobic organic compounds have many disadvantages and inefficiencies. For example, sequential processes require high economic investments. The quantities of additives suggested by known technologies are significant, the proportions of these additives result in using reactors that are too large, quantities of hazardous compounds that are too great, as well as significant investments, costs and security measures.
There is thus a need in the field to overcome at least one of the disadvantages of what is already known in the field.